Chapter 5: CPU Scheduling. 5.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts...

Chapter 5: CPU SchedulingChapter 5: CPU Scheduling

5.2 Silberschatz, Galvin and Gagne ©2005Operating System Concepts

Chapter 5: CPU SchedulingChapter 5: CPU Scheduling

Basic Concepts

Scheduling Criteria

Scheduling Algorithms

Multiple-Processor Scheduling

Real-Time Scheduling

Thread Scheduling

Operating Systems Examples

Algorithm Evaluation


Basic ConceptsBasic Concepts

Maximum CPU utilization obtained with multiprogramming

CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait

CPU burst distribution


Alternating Sequence of CPU And I/O BurstsAlternating Sequence of CPU And I/O Bursts


Histogram of CPU-burst TimesHistogram of CPU-burst Times


CPU SchedulerCPU Scheduler

Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them

CPU scheduling decisions may take place when a process:

1. Switches from running to waiting state

2. Switches from running to ready state

3. Switches from waiting to ready

4. Terminates

Scheduling under 1 and 4 is nonpreemptive

All other scheduling is preemptive


DispatcherDispatcher

Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves:

switching context

switching to user mode

jumping to the proper location in the user program to restart that program

Dispatch latency – time it takes for the dispatcher to stop one process and start another running


Scheduling CriteriaScheduling Criteria

CPU utilization – keep the CPU as busy as possible

Throughput – # of processes that complete their execution per time unit

Turnaround time – amount of time to execute a particular process

Waiting time – amount of time a process has been waiting in the ready queue

Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)


Optimization CriteriaOptimization Criteria

Max CPU utilization

Max throughput

Min turnaround time

Min waiting time

Min response time


First-Come, First-Served (FCFS) SchedulingFirst-Come, First-Served (FCFS) Scheduling

Process Burst Time

P1 24

P2 3

P3 3

Suppose that the processes arrive in the order: P1 , P2 , P3

The Gantt Chart for the schedule is:

Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17

P1 P2 P3

24 27 300


DefinitionsDefinitions

i

n of jobs

ii=1

T turnaround time;

duration of the j-th job

T global turnaround time ;

mean turnaround time ;

weighted turnaround time of the j-thjob ;

1

i

ii

i

t

TATG

TATGTATM

number of job

TTATP

t

TATMPnumber of j

n of jobs

i=1

weighted mean turnaround time;i

i

T

ob t


Example Example J=(7, 6.5, 2, 2, 2)J=(7, 6.5, 2, 2, 2)

Ti = (7, 6.5, 2, 4, 6)

TATG = 7+6.5+2+4+6 = 25.5

TATM = 5.1

TATMP = (7/7+6.5/6.5+2/2+4/2+6/2) = 8/5= 1.6

P1

P2

P3 P4 P5

0 1 2 3 4 5 6 7

3

2

1

Processor


Single processor systems Single processor systems - FIFO- FIFO

20001

tiArrivalP

100102

25253

P Arrival Begin End Ti TATPi

1 0 0 200 200 1

2 10 200 300 290 2.9

3 25 300 325 300 12

TATG = 790 TATM = 263 TATMP = 15.9/3 = 5.3

P3P1 P2

0 200 300 325


Shortest-Job-First (SJF) SchedulingShortest-Job-First (SJF) Scheduling

Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time

Two schemes:

nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst

preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF)

SJF is optimal – gives minimum average waiting time for a given set of processes


Process Arrival Time Burst Time

P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

SJF (non-preemptive)

Average waiting time = (0 + 6 + 3 + 7)/4 = 4

Example of Non-Preemptive SJFExample of Non-Preemptive SJF

P1 P3 P2

73 160

P4

8 12


Single processor systems Single processor systems - SJN- SJN

P Arrival ti

1 0 200

2 10 100

3 25 25


1 0 0 200 200 1

2 10 225 325 315 3.15

3 25 200 225 200 8

TATG = 715 TATM = 238 TATMP = 12.15/3 = 4.05

P3P1 P2

0 200 225 325

P3 P1P2

0 15025 35050

Lookhaed


Lookhaed algorithm Lookhaed algorithm

P Arrival ti

1 0 200

2 10 100

3 25 25


1 0 150 350 350 1.75

2 10 50 150 140 1.4

3 25 25 50 25 1

TATG = 515 TATM = 171 TATMP = 4.15/3 = 1.38

P3 P1P2

0 15025 35050


Multiprocessor systemMultiprocessor system - - J=(8, 5, 4, J=(8, 5, 4, 44, 2, 1), 2, 1)

P1

P2

P3

P6 P4

0 1 2 3 4 5 6 7 8 9 10 11 12

P5

3

2

1

Processore

P4

P2

P3

P6 P1

0 1 2 3 4 5 6 7 8 9 10 11 12

P5

3

2

1

Processore

P4

P2

P3

P6

P1

0 1 2 3 4 5 6 7 8 9 10 11 12

P5

3

2

1

Processore

SJN

Ad hoc

LJN


LJF is not optimalLJF is not optimal

P5

P2

P3

P1

0 1 2 3 4 5 6 7 8 9 10 11 12

3

2

1

Processore

P7

P4 P8

P6

P5P2

P3

P1

0 1 2 3 4 5 6 7 8 9 10 11 12

3

2

1

Processore

P7

P4 P8

P6

W(LJF) 4 11

W(OPT) 3 3m

W(SJF) 11 2

W(OPT) m

J=(8, 6.5,

6, 4, 3,

2.5, 2.5,

1)


Example of Preemptive SJFExample of Preemptive SJF

Process Arrival Time Burst Time

P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

SJF (preemptive)

Average waiting time = (9 + 1 + 0 +2)/4 = 3

P1 P3P2

42 110

P4

5 7

P2 P1

16


Preemptive SJFPreemptive SJF

P2

P3

0 1 2 3

2

1

P1

P2

P3

0 1 2 3

2

1

P1

P1

Scheduling without preemption Scheduling with preemption

W t tiii p i

p

min max max( )

1 1

, 1

c


Completion timeCompletion time

W t tiii p i

p

min max max( )

1 1

, 1

c

p : processes

c: processors

ti : completion time

Example:

Ja=(1, 2, 2) Wmin = 2.5

Jb=(1, 2, 4) Wmin = 4


Determining Length of Next CPU BurstDetermining Length of Next CPU Burst

Can only estimate the length

Can be done by using the length of previous CPU bursts, using exponential averaging

:Define 4.

10 , 3.

burst CPU next the for value predicted 2.

burst CPU of lenght actual 1.

1

n

th

n nt

.1 1 nnn t


Prediction of the Length of the Next CPU BurstPrediction of the Length of the Next CPU Burst


Examples of Exponential AveragingExamples of Exponential Averaging

=0 n+1 = n

Recent history does not count =1

n+1 = tn

Only the actual last CPU burst counts If we expand the formula, we get:

n+1 = tn+(1 - ) tn -1 + …

+(1 - )j tn -j + …

+(1 - )n +1 0

Since both and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor


Priority SchedulingPriority Scheduling

A priority number (integer) is associated with each process

The CPU is allocated to the process with the highest priority (smallest integer highest priority)

Preemptive

nonpreemptive

SJF is a priority scheduling where priority is the predicted next CPU burst time

Problem Starvation – low priority processes may never execute

Solution Aging – as time progresses increase the priority of the process


Round Robin (RR)Round Robin (RR)

Each process gets a small unit of CPU time (time quantum), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue.

If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units.

Performance

q large FIFO

q small q must be large with respect to context switch, otherwise overhead is too high


Example of RR with Time Quantum = 20Example of RR with Time Quantum = 20

Process Burst Time

P1 53

P2 17

P3 68

P4 24 The Gantt chart is:

Typically, higher average turnaround than SJF, but better response

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162


Time Quantum and Context Switch TimeTime Quantum and Context Switch Time


Turnaround Time Varies With The Time QuantumTurnaround Time Varies With The Time Quantum


Multilevel QueueMultilevel Queue

Ready queue is partitioned into separate queues:foreground (interactive)background (batch)

Each queue has its own scheduling algorithm

foreground – RR

background – FCFS

Scheduling must be done between the queues

Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility of starvation.

Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e., 80% to foreground in RR

20% to background in FCFS


Multilevel Queue SchedulingMultilevel Queue Scheduling


Multilevel Feedback QueueMultilevel Feedback Queue

A process can move between the various queues; aging can be implemented this way

Multilevel-feedback-queue scheduler defined by the following parameters:

number of queues

scheduling algorithms for each queue

method used to determine when to upgrade a process

method used to determine when to demote a process

method used to determine which queue a process will enter when that process needs service


Example of Multilevel Feedback QueueExample of Multilevel Feedback Queue

Three queues:

Q0 – RR with time quantum 8 milliseconds

Q1 – RR time quantum 16 milliseconds

Q2 – FCFS

Scheduling

A new job enters queue Q0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1.

At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.


Multilevel Feedback QueuesMultilevel Feedback Queues


Multiple-Processor SchedulingMultiple-Processor Scheduling

CPU scheduling more complex when multiple CPUs are available

Homogeneous processors within a multiprocessor

Load sharing

Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing


Real-Time SchedulingReal-Time Scheduling

Hard real-time systems – required to complete a critical task within a guaranteed amount of time

Soft real-time computing – requires that critical processes receive priority over less fortunate ones


Pthread Scheduling APIPthread Scheduling API

PTHREAD_SCOPE_SYSTEM

A thread that content with other processes and other PTHREAD_SCOPE_SYSTEM threads for the CPU.

If there is one process P1 with 10 threads and a single threaded process P2,

P2 will get one timeslice out of 11 and

every thread in P1 will get one timeslice out of 11.


Operating System ExamplesOperating System Examples

Windows XP scheduling

Linux scheduling


Pthread Scheduling APIPthread Scheduling API

PTHREAD_SCOPE_PROCESS

All threads of a process are grouped together and this group of threads contents for the CPU.

If there is a process with 4 PTHREAD_SCOPE_PROCESS threads and 4 PTHREAD_SCOPE_SYSTEM threads

each PTHREAD_SCOPE_SYSTEM threads will get a fifth of the CPU (4/5) and

the other 4 PTHREAD_SCOPE_PROCESS threads will share the remaining fifth of the CPU.


Solaris 2 SchedulingSolaris 2 Scheduling


Solaris Dispatch Table Solaris Dispatch Table


Windows XP PrioritiesWindows XP Priorities


Linux SchedulingLinux Scheduling

Two algorithms: time-sharing and real-time Time-sharing

Prioritized credit-based – process with most credits is scheduled next

Credit subtracted when timer interrupt occurs When credit = 0, another process chosen When all processes have credit = 0, recrediting occurs

Based on factors including priority and history Real-time

Soft real-time Posix.1b compliant – two classes

FCFS and RR Highest priority process always runs first


The Relationship Between Priorities and Time-slice lengthThe Relationship Between Priorities and Time-slice length


List of Tasks Indexed According to ProritiesList of Tasks Indexed According to Prorities

End of Chapter 5End of Chapter 5


Dispatch LatencyDispatch Latency


Thread PrioritiesThread Priorities

Priority Comment

Thread.MIN_PRIORITY Minimum Thread Priority

Thread.MAX_PRIORITY Maximum Thread Priority

Thread.NORM_PRIORITY Default Thread Priority

Priorities May Be Set Using setPriority() method:

setPriority(Thread.NORM_PRIORITY + 2);

Chapter 5: CPU Scheduling. 5.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts...

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Transcript of Chapter 5: CPU Scheduling. 5.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts...